The following abbreviations are herewith defined:
3GPP third generation partnership project
ACK acknowledge
ARQ automatic repeat request.
CQI channel quality indication
DL downlink (Node-B to UE)
DTX discontinuous transmission
FDD frequency division duplex
HARQ hybrid ARQ
HSDPA high speed downlink packet access
HS-DPCCH high speed dedicated physical control channel
HS-DSCH high speed downlink shared channel
HS-PDSCH high speed physical downlink shared channel
HS-SCCH shared control channel for HS-DSCH
MCS modulation and coding scheme
MIMO multiple input multiple output
NACK not acknowledge, negative acknowledge
Node-B base station
UE user equipment
UL uplink (UE to Node-B)
UMTS universal mobile telephony system
UTRA UMTS terrestrial radio access
3GPP RAN1 meeting #45 8.-12.5.2006 agreed to recommend specifying a 2×2 (dual transmit—dual receive antenna—dual code word) MIMO technique for HSDPA.
As is known in the art, MIMO only provides gains in good channel conditions, and thus a single stream transmission should be applied when the channel conditions are less than what will provide an adequate MIMO dual-stream transmission.
In addition to dual stream transmission the selected MIMO scheme was determined to be a dual code word system. This implies that each of the streams should have independent HARQ operation.
The HSDPA delivers downlink HARQ feedback (ACK/NACKs) in the UL on a HS-DPCCH physical channel for use by a Node-B packet scheduler. Based on the provided feedback Node-B the scheduler makes decisions whether to perform re-transmissions for the failed transmission or whether to transmit a new packet for each stream separately.
Referring to 3GPP TS25.212, V7.1.0 (2006-06), 3rd generation Partnership Project; Technical Specification Group Radio Access Network; Multiplexing and channel coding (FDD) (Release 7), section 4.7.1.1 describes the HARQ feedback as a 1-bit indication (ACK/NACK) that is (10,1) repetition coded.
Referring to 3GPP TS25.211, V7.0.0 (2006-06), 3rd generation Partnership Project; Technical Specification Group Radio Access Network; Physical channels and mapping of transport channels onto physical channels (FDD) (Release 7), section 5.2.1.2 “HS-DPCCH” defines the HS-DPCCH channel structure for delivering the coded bits to the Node-B in the UL.
As is described in section 5.2.1.2, the frame structure of the HS-DPCCH is defined in FIG. 2A (shown herein also as FIG. 2A). The HS-DPCCH carries uplink feedback signaling related to downlink HS-DSCH transmission. The HS-DSCH-related feedback signaling consists of Hybrid-ARQ Acknowledgment (HARQ-ACK) and Channel-Quality Indication (CQI). Each sub-frame of length 2 ms (3*2560 chips) consists of 3 slots, each of length 2560 chips. The HARQ-ACK is carried in the first slot of the HS-DPCCH sub-frame. The CQI is carried in the second and third slot of a HS-DPCCH sub-frame. There is at most one HS-DPCCH on each radio link. The HS-DPCCH can only exist together with an uplink DPCCH. The timing of the HS-DPCCH relative to the uplink DPCCH is shown in section 7.7 of 3GPP TS25.211.
The spreading factor of the HS-DPCCH is 256. i.e., there are 10 bits per uplink HS-DPCCH slot. The slot format for the uplink HS-DPCCH is defined in Table 5A, shown herein as FIG. 2B.
There is at least one inherent potential problem in the functioning of HARQ signaling. More specifically, the HS-DSCH transmission to a UE is indicated on the HS-SCCH. The HS-SCCH signaling for MIMO also indicates the number of applied streams. If the number of streams is detected incorrectly by the UE due to some reception error, the HARQ feedback in UL will reflect this reception error. In this case the UE would provide the HARQ feedback assuming wrong number of streams/code words. The Node-B might not have a means to detect such errors correctly unless the encoding of the HARQ feedback is designed to support that.
For example, if independent HS-SCCH signaling is provided for each HS-PDSCH (e.g., for each stream), the UE may detect only one HS-SCCH correctly. In such a case the UE would report a single ACK/NACK based on the detected stream.
If the Node-B is unable to detect this situation where the UE erroneously assumes only a single stream being transmitted, then unnecessary higher layer re-transmissions may occur since a detection miss would be erroneously interpreted as an ACK.
R1-062031, “HS-DPCCH in support of D-TxAA”, August 28th-September 1st, Qualcomm Europe, makes a proposal that assumes two independent ACK/NACKs (for each stream), or only a single ACK/NACK. However, no indication on the number of streams received is provided.
Reference can also be made to 3GPP TSG-RAN WG1, MIMO R7, Teleconference, Sep. 27, 2006, R1-MIMOR7-QCOM-09, ACK/NACK coding, Qualcomm Europe, which suggests the use of two separate (5,1) repetition block codes on the first ten bits of a HS-DPCCH TTI, and possibly interleaving the two blocks of (5,1) repetition codes.